Generally in the art a metal or glass substrate is roughened by some means before the first layer of coating is applied so that mechanical bonding will assist chemical adhesive means in holding the coating onto the substrate. Typical roughening means include acid etching, sand-blasting, grit-blasting, and baking a rough layer of glass, ceramic or enamel frit onto the substrate. The problem of adhesion of non-stick coatings to substrate is exacerbated by the nature of the coatings. If the coating is optimized for release to prevent food from sticking to it, for easy clean-up after cooking or durability, or to facilitate low friction sliding contact, almost by definition there will be difficulties in making it adhere to the substrate.
The substrate can be metal, often aluminum or stainless steel used for cooking or in industrial applications. It can be glass or ceramic. It might even be plastic for microwave oven cookware, or it could be an industrial article such as a saw made of carbon steel. Whatever the substrate or application, if it is necessary to roughen the substrate to make the coating adhere, that at least adds cost and can add other difficulties including creating a rough profile which can protrude or telegraph through the coating. This is especially undesirable when smoothness is sought, such as for saws, steam irons and copier rolls. The environmental cost of disposing of etchant materials can be significant. Sometimes, especially for glass and ceramic substrate, it can cause unacceptable weakness or brittleness of the substrate.
Means of enhancing adhesion of non-stick coatings to a substrate are illustrated in the following patents.
U.S. Pat. No. 4,049,863 (Vassiliou) discloses a primer containing fluoropolymer, such as polytetrafuoroethylene (PTFE), colloidal silica, and a polyamide imide (PAI), along with other constituents, applied by various techniques to a substrate that is preferably pretreated by grit-blasting, flame spraying of metals or metal oxides or frit coating, or to phosphated and chromated metals. The PTFE:PAI ratio can be 1:9. The primer coat is ordinarily applied at a thickness of 2-15 .mu.m. After air drying, the primer coat is top-coated with a conventional fluoropolymer enamel and baked. (Parts, percentages and proportions herein are by weight except where indicated otherwise.)
U.S. Pat. No. 5,230,961 (Tannenbaum) teaches concentration gradient coatings based on a primer which is applied to the substrate in the form of an aqueous dispersion comprising perfluorocarbon resin and at least one of polyamide imide, polyarylene sulfide and polyethersulfone resins where the perfluorocarbon resin comprises 50-90% by weight of a first resin of polytetrafluoroethylene having a melt viscosity of at least about 10.sup.10 poise (10.sup.9 Pa.multidot.s) plus 50-10% of a second resin of perfluorinated copolymer (FEP) of hexafluoropropylene and tetrafluoroethylene having a melt viscosity in the range of 10.sup.3 to 10.sup.8 poise (10.sup.2 to 10.sup.7 Pa.multidot.s) and a topcoat and any intermediate coats comprising perfluorocarbon resin.
Similar examples of aqueous blending of high and low melt viscosity resins are described in U.S. Pat. Nos. 5,168,107; 5,168,013; 5,223,343 and 5,240,775 (all to Tannenbaum) and U.S. Pat. No. 4,087,394 (Concannon).
Also, British Patent 1,454,255 (Berghmans & Seymus) discloses aqueous dispersion coatings of mixtures of PTFE and FEP with SiO.sub.2 and aluminum phosphate on preferably grit-blasted or frit-coated aluminum, but also on un-treated aluminum.
U.S. Pat. No. 4,287,112 (Berghmans) discloses PPS with PTFE, FEP and/or a co-polymer (PFA) of TFE and perfluoro(alkyl vinyl ether) (PAVE) as described in U.S. Pat. No. 4,292,859 and 4,351,883 (Concannon), along with aluminum flake and TiO.sub.2 pigment, in both aqueous and non-aqueous coatings. The preferred PTFE is irradiated micropowder. PTFE micropowder may be made according to the teachings of U.S. Pat. No. 3,116,226 (Bowers); 4,029,890 (Brown etal.) or 4,200,551 (Derbyshire).
Perfluoropolymer concentration gradient coatings obtained from non-aqueous formulations are also known. Disclosures in U.S. Pat. No. 3,661,831 and 4,143,204 are representative. A mixture of FEP and PTFE in an anhydrous organosol is also disclosed in U.S. Pat. No. 3,904,575 (Satokawa), which states that a higher stability of the organosol is generally obtained by a lower degree of polymerization of PTFE or a lower content of PTFE. The use of PAI and other film formers is also disclosed. The examples teach sand-blasting of an aluminum substrate before applying the coating.
Coatings systems including intermediate coats and topcoats are described in various patents including U.S. Pat. Nos. 4,049,863 (Vassiliou); 4,118,537 (Vary & Vassiliou); 4,123,401 (Berghmans & Vary); 4,252,859 (Concannon & Vary) and 4,351,882 (Concannon), all incorporated herein by reference.
Such blends of high and low melt viscosity fluoropolymers give an effective concentration gradient after baking the primer, achieving adhesion to the substrate without typical roughening means such as chemical etching, sand-blasting, or grit-blasting.
It would be advantageous to obtain the same effect without the necessity of blending fluoropolymer resins. Coatings, and coating formulations, based on a single perfluorocarbon resin are therefore desired. Non-melt-fabricable TFE polymers that will yield such improved coatings are also desired.
Tetrafluoroethylene (TFE) polymers are well known. The group of TFE polymers includes polytetrafluoroethylene (PTFE), and copolymers of TFE with such small concentrations of copolymerizable modifying monomers that the resins remain non-melt-fabricable (modified PTFE). The modifying monomer can be, for example, hexafluoropropylene (HFP), perfluoro(propyl vinyl ether) (PPVE), chlorotrifluoroethylene (CTFE), or other monomer that introduces side groups into the molecule. The concentration of such modifiers is usually less than 1 wt %, commonly less that 0.5 wt %.
PTFE and modified PTFE can be produced by the process known as dispersion polymerization, which typically yields an aqueous dispersion (raw dispersion) of small particles which can be coagulated and dried to obtain coagulated dispersion resin (also known in the art as fine powder) or concentrated and/or stabilized for use as a dispersion. Dispersions are useful for various purposes, including the coating of metal substrates. When non-melt-fabricable TFE polymer resin is used to make coatings on metal substrates, dispersions of PTFE having melt viscosity of the order of 10.sup.10 Pa.multidot.s are ordinarily used in complex formulations (paints) designed to enhance film formation and productivity in large volume commercial processes. These processes involve drying of wet coating deposit and subsequent fusion of the PTFE. With the high viscosity of prior art PTFE resins, fusion (baking) time is long and consolidation of the PTFE to a pore-free film is difficult to achieve.
Holmes & Fasig in U.S. Pat. No. 3,819,594 disclose a TFE fine powder resin consisting essentially of a copolymer of TFE with 0.03-1.0 wt % of perfluoro(alkyl vinyl ether) (PAVE), the resin having MV of from 1.times.10.sup.9 to 4.0.times.10.sup.10 poise (1.times.10.sup.8 to 4.0.times.10.sup.9 Pa.multidot.s), a standard specific gravity of no greater than 2.175, and a flex life of at least 8.times.10.sup.6 cycles after aging at 322.degree. C. for 30 days. Utility of such resin for coatings is not disclosed.
Morgan in U.S. Pat. No. 4,879,362 discloses a dispersion-produced non-melt-fabricable TFE copolymer having rheometer pressure of less than 24.1 MPa (3500 psi) (lubricated extrusion pressure as determined according to ASTM D-1457). Resin obtained by coagulating and drying the raw dispersion, in addition to having unusually low rheometer pressure, has low paste extruded green strength, and is non-agglomerating and non-fibrillatible. As such, it forms platelets on shear blending into elastomeric compositions instead of fibrillating. Utility of such resin for coatings is not disclosed.
The disclosure of the aforementioned patents, patent applications and publications are hereby incorporated by reference.